Patients' monthly visits during treatment and follow-up include a neurological examination to assess disease status; MRI to assess brain changes; and blood and urine collection to monitor liver, kidney and other functions. In addition, a lumbar puncture (spinal tap) is done during the last month of the baseline phase and one month after treatment ends to study changes in the spinal fluid surrounding the brain and spinal cord, and leukapheresis is done once during the last month of the baseline phase and once during the last month of treatment to collect white blood cells for study. These procedures involve the following: